We will investigate a novel microRNA (miR) mediated genetic pathway that is essential for normal craniofacial development. MicroRNAs are small, non-coding RNAs that repress gene expression post-transcriptionally. Using conditional inactivation in mice and chromatin immunoprecipitation (ChIP), we recently reported that the miR17-92 cluster is a direct target for Bmp signaling in cardiac progenitors. Our preliminary data indicate that miR17-92 mutant embryos have severe craniofacial phenotypes, including cleft lip and palate (CL/P) and mandibular hypoplasia, consistent with the hypothesis that miR17-92 is a Bmp-target in craniofacial morphogenesis. This is the first genetic evidence that an individual miR or miR cluster is functionally important in mammalian CL/P. Importantly, miR17-92 is located on human chromosome 13q31.3 in a critical region for cleft lip and palate associated with 13q deletion syndrome. Moreover, the miR17-92 cluster has been implicated in human cancers such as breast cancer, retinoblastoma, and lymphoma indicating that insight into miR17-92 function has broad human health implications. Our recently published findings indicate that miR17- 92 plays a critical role in the transition from progenitor to differentiated cll by downregulating progenitor genes such as the DiGeorge/velo-cardio-facial syndrome (DGS) gene, Tbx1. Our preliminary data suggest that miR17-92 also represses Tbx3, the ulnar mammary syndrome (UMS) gene. We propose to rigorously investigate the miR17-92 regulated pathways in developing craniofacial structures: In Specific Aim 1: We will investigate the hypothesis that miR17-92 inhibits the Tbox transcriptional regulator,Tbx3, to regulate lip and palate development. In Specific Aim 2: We will test the hypothesis that miR17-92, by precisely regulating Tbx1 expression levels, is a genetic modifier for Tbx1. In Specific Aim 3, we will investigate the hypothesis that miR17-92 seed sites in Tbx1 and Tbx3 directly inhibit Tbx1 and Tbx3 spatiotemporal expression during in vivo midface development. There is poor understanding of the genetic mechanisms underlying CL/P. New genetic insights will be critical for diagnostic testing and family counseling in the future. Moreover, an in depth knowledge of genetics of CL/P will provide critical resources for patient management as human genome sequencing becomes more commonplace. Finally, there is the long term goal to develop novel therapeutic strategies based on solid scientific information that will come from work in model organisms and human genetics.